(a) Field of the Invention
The present invention relates to purification of an exhaust gas of a diesel engine vehicle. More particularly, the present invention relates to a method and apparatus for controlling regeneration of a simultaneous NOx-PM reduction apparatus having a lean NOx trap (LNT) and a catalytic particulate filter (CPF).
(b) Description of the Related Art
Nitrogen oxides (NOx) and particulate matters (PM) are principal air pollution material of a diesel engine vehicle. Therefore, they are under strict emission regulation of a diesel engine.
Several technologies have been developed to cope with the emission regulation of the diesel engine. For example, fuel injection timing may be delayed. In addition, exhaust gas recirculation (EGR) has been enhanced such that NOx exhaust may be reduced. In addition, combustion characteristic of the diesel engine has been improved so as to reduce PM.
Such technologies for complying with the diesel engine emission regulation may be classified as an engine improving technology and a post-processing technology. Examples of the post-processing technology for reducing the principal exhaust gas of a diesel engine are (1) oxidation catalyst for purifying high boiling point hydrocarbon (HC) among the particulate matters, (2) a DeNOx catalyst for decomposition or reduction of the NOx under an excess oxygen atmosphere, and (3) a diesel particulate filter (DPF) system filtering the PM.
Among such a post-processing technology, a simultaneous NOx-PM reduction apparatus having a lean NOx trap (LNT) and a catalytic particulate filter (CPF) is also included. Regarding the simultaneous NOx-PM reduction apparatus, it is believed that simultaneous generation of the LNT and DPF is not possible. In addition, the regeneration of LNT is required to be performed in a rich range of an air/fuel ratio, and thus the regeneration of the LNT causes a substantial amount of PM.
The DPF system has merit in that PM is sufficiently trapped so as to substantially decrease an exhaust of the PM. However, apparatus and/or algorithm for a regeneration control of the trapped PM by an after-burning thereof have not been sufficiently investigated. Consequently, in current systems, durability of the DPF is still problematic since an excessive heat may be produced during the regeneration of the filter thereby causing damage of the DPF. In addition, during a large scale of EGR for removal of the NOx, a substantial amount of the PM is produced. Therefore, in this case, a PM trapping limit of a catalyst support of the DPF easily becomes saturated, and this may be a cause of the damage of the support of the DPF during an active regeneration of the DPF.
In particular, according to the conventional regeneration method, an interior temperature of the DPF is maintained at about 550° C. by raising the exhaust gas temperature by fuel post-injection of the engine, and the trapped PM in the DPF is burned by the heat of the exhaust gas. In addition, the regeneration of the DPF is performed under a regeneration condition that is not dependent on trapped PM amount. However, when the interior temperature of DPF is failed to be strictly controlled under 1,000° C., a SiC support may be damaged by an abrupt increase of the interior temperature of DPF abruptly increase. Therefore, conventionally, even if the filter of SiC material has a PM trapping capacity of 10 g/l, the DPF regeneration is performed under a condition of less than 6 g/l for safety and durability. In addition, the interior temperature of DPF is controlled to be lower than 700° C.
However, from such a mild regeneration, only a relatively low DPF regeneration efficiency may be achieved. In addition, in this case, a central area of the DPF is intensively regenerated, and accordingly PM usually remains at a marginal area thereof. Therefore, after a repeated regeneration, the PM remaining at the marginal area of the DPF suddenly exceeds the SiC regeneration limit and in this case, support damage may be caused by an uncontrolled combustion. In addition, an enhancement of the DPF regeneration efficiency and prevention of a deterioration of fuel consumption may be achieved by increasing the interior temperature of the DPF. When the post-injecting fuel amount is increases or injection timing is adjusted in order to the increase of the interior temperature of the DPF, the LNT disposed in front of the DPF tends to be easily degraded.
Due to such a defective possibility, even if the CPF may have the PM trapping capacity of 10 g/l, the regeneration is performed when the trapped PM amount reaches about 4-5 g/l, which is equal to or less than a half of the trapping capacity. Therefore, the regeneration is frequently performed, and the fuel consumption is substantially deteriorated.
The information set forth in this Background of the Invention section is only for enhancement of understanding of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.